In this specification, where a document, act or item of knowledge is referred to or discussed, this reference or discussion is not an admission that the document, act or item of knowledge or any combination thereof was at the priority date part of common general knowledge, or known to be relevant to an attempt to solve any problem with which this specification is concerned.
In digital radio frequency transmission systems, the packets of user data to be transmitted are usually first encoded using a forward error-correction code such as a Turbo code. The encoded packets are then modulated as data symbols onto a carrier wave using one of a multitude of modulation schemes, such as quadrature amplitude modulation, binary phase shift keying or quadrature phase shift keying, and then transmitted to a receiver.
The receiver then processes the signal by demodulating the data symbols and decoding the encoded bits to thereby retrieve the user data. To assist in the efficient processing of the signal, the receiver estimates certain parameters of the channel over which the modulated carrier wave was transmitted in a process referred to as ‘acquisition’, involving analysis of the demodulated data symbols to estimate channel parameters before decoding is commenced.
Pilot symbols, with selected values that are known or can be precisely determined by the receiver, are conventionally included in the packets to assist the receiver in channel acquisition. Each packet may, for example, include a preamble (or postscript) comprising a sequence of pilot symbols (or ‘unique word’, UW) able to be readily analysed by the receiver and the result compared with known pilot symbol values stored in the receiver. This allows an accurate estimation of channel parameters such as the offsets of phase, time and frequency from an expected value. Mobile satellite communications can be subject to many noise effects as well as to fading effects, and the judicious use of pilot symbol techniques can significantly assist in minimising the problems associated with such effects. However, such methods can add considerable complexity to the signal acquisition and decoding process, resulting in slow performance and hence transmission delays. Excessive delay is incompatible with real time applications such as telephony and videoconferencing.
Examples of the use of such contiguous block preamble pilot symbol sequences in channel acquisition are discussed in U.S. Pat. Nos. 6,778,586, 6,711,208 and 6,625,459. In addition, U.S. Pat. No. 6,782,037 describes the use of such pilot symbol sequences to assist in distinguishing between users.
In an alternative technique, such as that described in U.S. Pat. No. 6,665,361, the pilot symbols are evenly distributed amongst the data symbols of each packet, rather than a placed as a block header at the beginning of the packet. This distribution scheme is said to assist in more accurate signal frequency and phase estimation, as well as allowing the tracking of a fading signal.
More recently, a technique for irregular fractal placement of pilot symbols amongst data symbols has been described in applicant's International Patent Application No. WO 2004/04929. Such a pilot symbol distribution allows for the efficient acquisition of the data packet's time of arrival and frequency, in a way that is particularly robust to phase noise. This relates to the fact that the pilot symbols, though irregularly spaced, are placed with a uniform density throughout the packet. Importantly, the pilot symbol pattern allows the spectrum of the pilot symbols to be narrower than that of the packet, thus reducing adjacent channel interference during acquisition.
In addition to information about channel parameters, a receiver may also require information about other parameters related to the signal in order for the signal to be efficiently processed. For example, knowledge of transmit parameters, such as the modulation scheme used to encode the data, type of (or rate of) forward error correction coding, user identification, transmitter location and power and type of antenna, can all assist the receiver to more efficiently process the received signal.
In the past, transmit parameters have been communicated to a receiver in a variety of ways. For example, in the case of code division multiple access (CDMA) systems, a user identification number is communicated to the receiver by allocating each user a mutually orthogonal unique code, whose ‘adding together’ in air can be detected by the receiver. Whilst CDMA systems are successful in allowing a receiver to identify multiple users who may be simultaneously accessing a single channel, only a relatively small amount of information can be encoded in this way. This means that it is not possible using CDMA to use such a technique to readily communicate to a receiver transmit parameters other than, say, user identification numbers.
Other approaches to this problem are discussed in U.S. Pat. No. 5,369,378, in which the phase-shift between adjacent pilot symbols is used to communicate information, and in EP-1 381 180, which describes a modulation scheme which can be varied in accordance with a parameter to be transmitted.
In other techniques, transmit parameters are communicated to the receiver in the actual values of the pilot symbols. U.S. Pat. No. 6,643,275 describes a system in which user identification information and the coding scheme is communicated to the receiver by way of the particular selection of pilot symbol preamble.
A more sophisticated system is described in International Patent Application No. WO 03/063405, in which the forward error-correction code rate is varied for each block of transmitted data. The actual value of the code rate of one of the blocks is communicated in a pilot symbol preamble, with that block containing data indicating the coding rate of a subsequent one or more blocks.
However, as with the CDMA approach discussed above, only a limited number of unique values can be communicated in this way, which consequently limits the type of information that can be meaningfully transmitted. Accordingly, it would be advantageous to provide an improved method and system for communicating information in a digital signal.